selftests: rtnetlink: Fix do_test_address_proto()

This selftest was introduced recently in the commit cited below. It misses
several check_err() invocations to actually verify that the previous
command succeeded. When these are added, the first one fails, because
besides the addresses added by hand, there can be a link-local address
added by the kernel. Adjust the check to expect at least three addresses
instead of exactly three, and add the missing check_err's.

Furthermore, the explanatory comments assume that the address with no
protocol is $addr2, when in fact it is $addr3. Update the comments.

Fixes: 6a414fd77f61 ("selftests: rtnetlink: Add an address proto test")
Signed-off-by: Petr Machata <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/rtnetlink.sh

···
 
 	count=$(address_count)
 	check_err $?
-	(( count == 3 )) # $addr, $addr2 and $addr3
+	(( count >= 3 )) # $addr, $addr2 and $addr3 plus any kernel addresses
+	check_err $?
 
 	count=$(address_count proto 0)
 	check_err $?
-	(( count == 1 )) # just $addr2
+	(( count == 1 )) # just $addr3
+	check_err $?
 
 	count=$(address_count proto 0x11)
 	check_err $?
-	(( count == 2 )) # $addr and $addr2
+	(( count == 2 )) # $addr and $addr3
+	check_err $?
 
 	count=$(address_count proto 0xab)
 	check_err $?
-	(( count == 1 )) # just $addr2
+	(( count == 1 )) # just $addr3
+	check_err $?
 
 	ip address del dev "$devdummy" "$addr"
 	ip address del dev "$devdummy" "$addr2"